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Old 17th July 2006, 05:08 AM   #1
mudihan is offline mudihan  United States
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Default Is my amp in danger?

I have been using OPA637s in the input buffer stage of my NHTPro A10 power amp (150W per channel, dual mono). The result is amazing as the sound has been vastly improved. However, the buffer stage is set at unity gain -- the OPA637 are not supposed to be stable at this lower gain. But I can't hear anything wrong. Everything is so smooth and clean. Is it because the 56p bypass capacitors that may have solved the problem of oscillation? Or are my OPA637s timed bombs? Thanks!
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Old 17th July 2006, 05:13 AM   #2
mudihan is offline mudihan  United States
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Schematic
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Old 17th July 2006, 05:15 AM   #3
mudihan is offline mudihan  United States
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Can't upload the schematic
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Old 17th July 2006, 05:18 AM   #4
mudihan is offline mudihan  United States
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never mind
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Old 17th July 2006, 05:45 AM   #5
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I've tried OPA637 in the I/V of DAC. It becomes oscilator. You touch the body of OPA637, is it hot? If it is, then you should look under osciloscope if there is oscilation when this chip is used as buffer.
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Old 17th July 2006, 05:50 AM   #6
mudihan is offline mudihan  United States
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They are cool even after hours of playing.

So, as long as oscillation is not present, the opamp is in stable condition?
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Old 17th July 2006, 06:35 AM   #7
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I'd not disregard manufacturers recommendations. You must have simply
gotten lucky with stability with a random sample of parts. Do you really want
to trust your power stage and speakers to this?

I really like the AD826 awhole lot... This device is unity-gain stable and is, in fact,
designed with the intentions of being a buffer. It can drive 50 ohm impedances
with tens of mA of current. Further, the op-amps output stage is Class AB, so,
for small signal excursions, the op-amp is running Class A. People like the
AD826. It's a $5 part too.

-- Jim
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Old 17th July 2006, 07:19 AM   #8
mudihan is offline mudihan  United States
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I like AD826 a lot too. It's different from BB chips in a very confident way. And I think it sonically it outperforms many other AD chips, such as OP270, OP275, AD823, and AD827. However, I found OPA637 to sound best in my setup. The only things that came remotely close were AD843 and LT1364.

I actually have about 4 pairs of OPA637s (mounted on Browndog adapters) and all of them SEEM (they all sound great with no distortion or over-heating deteted) to be stable. (On the other hand, my pairs of OPA627 seem to get warm easily). It's just weird.

Again, something I really need to confirm:

Is oscillation the cause of instability? If the chip sounds great and is not over-heating, does this mean that there is no oscillation (or, what are the signs of oscillation -- I don't currently have a osciloscope)?

Many thanks!
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Old 17th July 2006, 01:54 PM   #9
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Quote:
Originally posted by mudihan
Is oscillation the cause of instability? If the chip sounds great and is not over-heating, does this mean that there is no oscillation (or, what are the signs of oscillation -- I don't currently have a osciloscope)?

Many thanks!
Oscillation is the result of instability.

Scopes are cheap. A decent old super-reliable Tek 465 can be had for perhaps $350 (often less than that), and if you're fooling with DIY, a scope is not optional gear.
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Old 17th July 2006, 03:58 PM   #10
mudihan is offline mudihan  United States
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Well, $350 is a bit too much for me. But your advice is well taken.
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